The present invention relates to vacuum pumps of the type known as diffusion pumps.
Diffusion pumps are well known and widely used for the attainment of high and ultra high vacuum. When used with modern working fluids and accessories, they can produce pressures approaching 10.sup.-10 mbar. The pumps are generally incapable of exhausting directly to the atmosphere and require the use of a backing pump, commonly an oil sealed rotary vacuum pump, in conjunction with the diffusion pump itself.
Diffusion pumps generally comprise a substantially cylindrical outer body which is cooled by, for example, coils helically wound around the outside of the body through which cooling water can be circulated or alternatively by air cooled fins attached to the outside of the body.
Within the outer body is positioned a hollow "chimney" sitting at, or close to, the base of the outer body and which tapers (continuously or, more usually, in stages) upwardly from the base. The chimney is generally contained within the outer body and is positioned substantially concentrically therein.
Across the top of the chimney but not in contact therewith is a top cap having a generally circular portion of somewhat larger diameter than the top of the chimney and positioned symmetrically with regard to the chimney and having a downwardly projecting annular side portion whose lower edge is somewhat beneath the upper edge of the chimney. The top cap is therefore substantially an inverted "cup shaped" component positioned about the top of the chimney with a circular or annular passageway therebetween. The top cap and the adjacent portion of the chimney defining the annular passageway is known as a `jet stage`. A heater is provided adjacent the base of the outer body and, prior to use of the pump, a working oil is placed in the base of the body to a height above the lower edge of the chimney positioned within the body. The oil is usually a low vapour pressure oil although some versions of diffusion pumps use mercury.
A pump inlet is positioned in the outer body above the top cap and an outlet in the side of the body towards the base of the chimney but above the oil level. The outlet is normally connected to a backing pump as described above.
In use of pumps of this type, the backing (rotary) pump is turned on and left running continuously, a pressure of at least 0.1 mbar being required on the exhaust side of the diffusion pump. The cooling system (water or air) for the outer body is turned on and the oil can now be heated by the heater for, for example, fifteen to twenty minutes, when it begins to boil. Hot vapour rises up the chimney and forms (aided by the taper) a relatively high oil pressure at the top of the chimney. The vapour is then urged through the passageway between the chimney and the top cap, the jet stage, to an area of much lower pressure and creates an annular vapour jet. This jet is designed to move at a velocity which is supersonic and which impinges on the inside surface of the cooled outer body where the vapour condenses and condensed oil flows down the inside wall of the outer body and returns to the oil reservoir at the base of the body.
With the diffusion pump turned on, gas molecules being pumped in to the inlet of the diffusion pump are likely to collide with the much heavier oil vapour molecules and be provided with a velocity component which will direct the gas molecules towards the outlet of the diffusion pump where they will be subsequently removed from the diffusion pump via the backing pump. A pressure difference is thereby established across the continuously flowing vapour jet.